From an upstairs window at the back of my house I can just about see the peaks of Docklands, where worried, tight-lipped, employees of Lehman Brothers declined to comment to BBC reporters this morning as the walls of global capital came tumbling down. I see no leaping from ledges. Perhaps, though, there's been a rush for London City Airport and cheap one-way tickets to Timbuktu – ok, Dundee. If so, this mass, er, flight from financial catastrophe will have come a little early to be aided by what appears to be a breach of a campaign promise by Boris Johnson.

Last week the London mayor revealed to the London Assembly that he was giving the "green light" to an expansion of the airport's activity, enabling it to increase the number of flights by up to fifty percent – from 80,000 to 120,000 a year – by 2010. This meant he had declined to use his powers to prevent Newham Council from granting the necessary planning consent, certain reassurances having been obtained.

This seems to contradict what Johnson said at an early mayoral hustings, held by the Green Alliance on 14th February. Asked by chair Jon Snow to summarise their position on the airport's future Ken Livingstone said he'd close it but lacked the powers. He would, however, prevent further development by blocking the sale of surrounding land. Johnson said he too would freeze expansion and encourage people to use trains instead. Click here and find my (slightly muddy) recording of the events from about a fifth of the way in.

Is there any way this isn't a U-turn? In a letter to the airport's managers written before giving his blessing to the plans, Johnson warned that any additional future expansion would be "extremely difficult" to back, could his backing away from his "freeze" pledge be mitigated as only a mini-thaw? Greens and other environmentalists are not impressed. I'm contracting the mayor's office for a response and will update when it's received.

UPDATE: The mayor's office has got back to me with the following statement:

"Because the plans for the expansion of City Airport predate the creation on the GLA as a statutory planning authority, Newham informally consulted the Mayor on the latest proposals in the summer. Although generally supportive of City Airport expansion plans, the Mayor wrote to Newham Council asking them to defer their decision until he had received further information on the impact of increased flight movements on areas around the airport earmarked for regeneration. He also wanted to await the results of a study of any potential risks to the proposed the Thames Gateway Bridge. The results from the study and the information provided by Newham have now answered the issues he raised."

Two points here. One is that Mayor Johnson's powers to intervene in the decisions of boroughs turned out not apply in this case because the airport submitted its plans to Newham Council before the GLA came into existence. The other is that he is "generally supportive" of the present expansion plans. This makes you wonder if he'd have imposed the "freeze" on expansion he pledged at the Green Alliance hustings even if he could have.

Would he act to block any further expansion plans submitted by the airport in future? His office sent me the text of letter referred to above (which Johnson sent to Newham Council, not to the airport's managers as had been misreported elsewhere) suggesting that he might well do. A more sceptical conclusion would be that a mayor deeply committed to environmental causes would not have declared himself "generally supportive" even of the present plans.